There is growing concern over the presence of contaminants such as pesticides, organic pollutants, pharmaceuticals and personal care products (PPCPs), and pathogens in source water for treatment plants, since such contaminants may pose serious negative environmental and human health effects. For example, high levels of pesticides in drinking water can cause long-term health effects, such as cancer or organ and reproductive system damage. Additionally, certain organic pollutants can also cause circulatory, nervous, reproductive and immune system deficiencies, organ damage, and cancer, which are serious long-term health effects. With respect to environmental impact, some pollutants are a nutrient for algae and result in increased rates of eutrophication, leading to reduced levels of oxygen in the water, and eventually wildlife death.
Current water treatment system components such as activated carbon, flocculation, and chlorine-based disinfection may also have harmful side effects on water quality since certain water disinfection byproducts may themselves be a cause for concern. Such problems are compounded in rural areas where contamination of source water is complicated by leaking septic tanks, pesticide and herbicide runoff from farms, and runoff of significant amounts of organic contaminants and animal waste from feedlots and ranches. Therefore the removal/inactivation of pesticides, organic pollutants, PPCP's and pathogens from drinking water systems is a priority.
Effective technology should have a minimum number of treatment steps, a minimum required amount of maintenance and oversight of the treatment process, low capital and operating costs, insensitivity of the treatment process to changing conditions or raw water quality, and ability of the treatment process to decontaminate the targeted pollutants without the formation of byproducts that are more hazardous than the parent compound.
Plasma-initiated oxidation and disinfection is considered an advanced oxidation technology and is characterized by the production of high oxidation potential species such as hydroxyl radical (.OH), hydrogen peroxide (H2O2), singlet oxygen (.O), and high-energy electrons, as examples. Such oxidizing species react readily with biological materials causing permanent damage, as well as with organic molecules, degrading them to CO2, water, and inorganic salts in many cases. However, existing plasma systems have been considered to be too complicated for use with small water treatment systems in addition to requiring costly and fragile equipment and having high operation costs.
One such technology that uses a plasma as the medium in which electric energy is transferred into a liquid to degrade organic compounds and promote microbial disinfection is the dense medium plasma reactor (DMPR) described in U.S. Pat. No. 5,534,232 for “Apparatus For Reactions In Dense-Medium Plasmas” which issued to Denes et al. on Jul. 09, 1996. Liquid/vapor phase chemicals are caused to react in a low-temperature plasma. Advantages of this type of reactor, when compared with conventional plasma processing reactors, are that chemical reactions occur at atmospheric pressure and approximately room temperature, and the plasma discharge is controlled by electron flux rather than thermionic emission. The DMPR emits a higher current, continuous discharge at lower voltages than previous aqueous plasma discharge reactors, reduces mass transfer limitations inherent to non-thermal plasma reactors, and influences aspects of the chemistry occurring in and on the surface of the plasma discharges.
While the DMPR is superior to known nonthermal, plasma oxidation technologies, fluid is inefficiently cycled into the plasma zone, thereby inhibiting electron-impact dissociation reactions.
Computational fluid dynamics simulations of fluid flow in dense medium plasma reactors and experiments performed using the DMPR have been incorporated into the Tubular High-Density Plasma Reactor described in “Development Of A Tubular High-Density Plasma Reactor For Water Treatment” by Derek C. Johnson et al., Water Research 40 (2006) pages 311-322. To maximize the oxidation pathway which results in a direct conversion of organics to CO2, it is proposed that the flow of contaminated liquid be made perpendicular to the plasma discharge; to decrease the voltage required to initiate the plasma from that of required for the DMPR by further lowering the pressure, gas may be introduced through the center of the discharge electrodes; and to prevent uneven wear on the discharge electrodes, which was found to be a problem for the DMPR, each electrode may have the same radial distance and thus the same angular velocity. Gas and liquid flow patterns as well as the plasma discharge path from the discharge electrodes (pins) to the outer electrically conducting cylinder through the passing contaminated aqueous solution are described in Johnson et al., supra.
The tubular high-density plasma reactor is expected to maximize the fraction of fluid in the annular gap contacting the plasma, thereby achieving an improvement over the DMPR, wherein approximately 90% of the fluid bypasses the plasma as it moves through the reaction zone.